1. Field of the Invention
This invention relates to jet printing inks, and to the use of polymerized fatty acid-based polyamide resins therein.
2. Description of the Related Art
There is a growing market for all manner of posters, advertising displays, banners, and the like printed by digital methods. One of these, impulse printing, also called drop-on-demand (DOD) inkjet printing, with low-volatility organic solvent-based ink is presently used by digital imaging equipment sold by Oce, Gretag, NUR and others to produce images with better color, adhesion, and abrasion-resistance than those images produced with water-based inks. Inkjet printing is performed by discharging ink droplets from a print head to a substrate. The droplets are ejected through orifices or nozzles in the faceplate of the print head and are directed to the substrate to form an image thereon. In contrast to many other types of printing, there is no contact between the printer and the substrate in inkjet techniques.
Most of the inkjet printers known in the art may be characterized as either continuous or impulse devices, depending upon the mechanism by which the ink droplets are directed to the substrate. In continuous inkjet systems, an essentially uninterrupted stream of ink is ejected from a nozzle and breaks up into droplets. The droplets bear an electric charge so that they can be deflected by an applied electric field which is modulated according to the particular image to be recorded. The electric field directs the droplets toward either the substrate or an ink re-circulating reservoir. The inks employed in conjunction with continuous inkjet systems typically comprise a colorant such as a dye or pigment, an electrolyte to facilitate droplet deflection, and a liquid vehicle to dissolve or disperse the colorant and the electrolyte. While the vehicle in many continuous-type inks comprises water, U.S. Pat. No. 4,142,905, in the name of Cooke, disclosed a water-free ink composition consisting essentially of a low molecular weight glycol, an inorganic salt electrolyte, and a dye which is soluble in the glycol.
With so-called “impulse” or “drop-on-demand” inkjet printers, image formation is controlled by selectively energizing and de-energizing a piezoelectric transducer rather than by modulating an applied electric field. Ink is stored in the print head or nozzle until it is necessary to form an image on the substrate. The printer is then activated to apply pressure to the ink and discharge a selected number of discrete ink droplets toward the substrate. These ink droplets need not, and typically do not bear an electric charge. Accordingly, impulse ink compositions are free of corrosive substances such as water and electrolytes which continuous stream inks often contain.
However, DOD inkjet printers present a number of problems that are not encountered in continuous inkjet (CIJ) systems. For example, unlike CIJ printers, impulse printers typically are maintained in a stand-by or temporarily inoperative mode between printing cycles. Thus, the ink is allowed to stand and possibly solidify in the discharge orifices of the print head. Impulse printers normally begin a printing cycle with such material in place. Many of the start-up problems encountered with impulse printers are attributable to ink which has been allowed to stand in the discharge orifices during stand-by periods. Such material is less of a concern in continuous systems because there typically are fewer interruptions in the flow of ink. Even where ink is allowed to stand and solidify, it is more easily purged due to the considerably higher pressures at which CIJ printers operate. Accordingly, DOD inks must be specially formulated to minimize start-up problems.
Although numerous ink compositions for DOD inkjet printers are known in the art, many of these inks are not suitable for writing on non-porous substrate other than vinyl. Some lack adhesion to polyolefin films, and some are hygroscopic leading to poor wetting and drying. To improve the adhesion of an ink requires use of an improved binder for the colorant in the ink or use of more (by weight) of more of a conventional binder. Yet to improve the economics of DOD printing, jobs must be printed faster which in turn requires inks that dry faster than many of the inks described in the prior art. Employment of a rapidly evaporating ink solvent is not necessarily the answer. Rapid evaporation of the vehicle of a volatile ink often leads to clogging of discharge orifices because the last portion of the ink ejected from an orifice collapses back onto the faceplate while the rest of the ink droplet breaks away and flies to the substrate. If the ink that falls on the faceplate dries before it can flow back into the orifice, a pile of binder resin and colorant will form at the edges of the orifice, gradually changing the way drops are discharged and even causing the orifice to clog entirely. Rapid evaporation of the vehicle can also clog the orifice during stand-by periods since the ink forms a meniscus at the air-ink interface and can, as solvent is released to the air, harden. These problems are aggravated when the ink formulator attempts to increase the binder content to improve adhesion of the ink on the substrate. Moreover, rapid evaporation is undesired due to the adverse environmental and health effects of many commonly employed ink vehicles. Finally, if the flash point of the ink is too low, generally below about 61° C., it cannot be shipped by air.
Printing inks for flexographic printing on non-porous substrates such as polyethylene and polypropylene are well known. Those formulated with a pigment, a fast-evaporating organic solvent and a resinous binder represent the state-of-the-art in packaging printing, allowing printing of high quality full-color images having excellent adhesion and abrasion-resistance even when the package is stored in a freezer for extended periods of time.
The organic solvent in these inks is typically a blend rich in a lower alcohol, for example, ethanol, propanol, or isopropanol, since these alcohols are good solvents for the resinous binder and evaporate rapidly after printing. Flexographic inks may also contain lesser amounts of a lower aliphatic ester, such as ethyl acetate and propyl acetate, and an aromatic solvent such as toluene. A common resin is the reaction product of a polymerized fatty acid (also referred to as dimer acid), a diamine or mixture of diamines, and a terminating monocarboxylic acid. These polymerized fatty acid-based polyamide resins suitable for use in flexographic inks are well known and described in detail in numerous patents as are methods for preparing the inks (e.g., U.S. Pat. Nos. 3,253,940, 3,622,604, 3,700,618, and 4,508,868). These patents make clear that polymerized fatty acid-based polyamides tend to form gels when dissolved in lower aliphatic alcohols. Indeed, a large proportion of the issued patents state that they are directed to resin compositions with improved solution storage stability. Because of these largely successful developments, the stability of commercial flexographic inks is not currently a problem. Nevertheless, no general theory to explain solution stability has been developed; that is, resins are developed empirically and test results for ethanol and propanol cannot be extended to other solvents.
These same patents do not discuss the need for a new kind of ink suitable for use in drop-on-demand (DOD) inkjet printers. DOD jet inks must employ a solvent with a much slower evaporation rate than the lower alcohols so that the nozzle of the printer does not clog. The inks must also have a much higher flash point than the lower alcohols so they can be transported by air and used in an office or small print shop environment safely.
There is also an extensive prior art disclosing the preparation of jet inks. A large proportion of said art teaches inks suitable for use in so-called thermal inkjet printers. These inks are largely water-based and contain no resin. Of the non-aqueous inkjet patents, a large proportion relate to so-called “continuous inkjet” or CIJ inks. These inks are based on low flash point, high volatility solvents such as methanol, acetone, ethanol, propanol, ethyl acetate, and methyl ethyl ketone. These inks have been tried in DOD printers but are detrimental to them since they dry rapidly and tend to clog the heads.
U.S. Pat. No. 4,207,577 (1980) discloses a CIJ ink composition that necessarily includes a cellulosic resin, an optional co-resin, a colorant, and at least one solvent. The necessary cellulosic component can be cellulose derivatives such as cellulose esters (nitrocellulose, cellulose acetate and the like), and especially cellulose ethers, for example, methylcellulose, hydroxyethylcellulose, hydroxypropylcellulose, propionitrile cellulose, ethyl cellulose, and benzylcellulose. This patent is one of the earliest patents describing a solvent inkjet ink.
U.S. Pat. No. 5,104,448 (1992) discloses low volatility and low viscosity jet printing inks consisting essentially of a dye (or pigment) and an optional binder, dissolved (or dispersed) in a polyhydric alcohol ether solvent (such as polypropylene glycol monoalkyl ether, ethylene glycol butyl ether, diethylene glycol butyl ether, etc.). This patent mentions that the disclosed ink composition is suitable for DOD printing.
U.S. Pat. No. 5,154,761 (1992) discloses DOD-type inkjet inks which produce well-defined images on porous substrates such as Kraft paper and corrugated cardboard. These ink compositions comprise from 65–80% of a low molecular weight diol, from about 15–25% of a low molecular weight diol-ether, and from about 1–15% of a colorant. Said inks contain no resin and are not suitable for printing on non-porous substrates.
U.S. Pat. No. 5,594,044 (1997) discloses a jet ink composition suitable for printing images which have good adhesion and isopropyl alcohol rub resistance, comprising an organic solvent, a urethane resin, a colorant, a cellulose nitrate resin, and an adhesion promoter such as a titanate. Organic solvents suitable for the ink composition of the invention include ketones, esters, C1-6 alcohols, hydrocarbons, glycols, and glycol ethers.
U.S. Pat. No. 5,663,217 (1997) discloses formulation of a jet ink which comprises a suspension in a non-aqueous solvent of a disperse phase containing an insoluble colorant, a resinous dispersant for the colorant, and a polymer binder wherein said solvent is a polyhydric alcohol ether. A new component disclosed in this patent is the resinous dispersant, selected to be capable of forming a solution in the chosen solvent and to disperse the dyestuff in the solvent so as to form a substantially stable dispersion.
U.S. Pat. No. 5,843,219 (1998) discloses inks based on petroleum distillates. Said inks are stable dispersions comprising pigment, diluent, and a dispersant for stabilizing the pigment in the medium, wherein the major component of the diluent is a hydrocarbon while the minor component is a polar component selected from alcohols, glycols, polyglycols, ethers of glycols and polyglycols, alkoxylated alkyl phenols and admixtures thereof.
U.S. Pat. No. 5,888,287 (1999) discloses an inkjet composition having a reduced tendency to smear upon application to a substrate. The ink composition contains no resin and is free of monohydric alcohol solvent.
U.S. Pat. No. 5,936,027 (1999) discloses a jet ink containing a colorant; a non-aqueous solvent comprising alcohol and ketone, either alone or in admixture; and resin comprising ethyl cellulose, phenolic resin, and silicone resin. While perhaps suitable for the CIJ printing method, these inks are likely too volatile for use in DOD devices.
U.S. Pat. No. 5,990,197 (1999) discloses an organic solvent-based polyester ink formulation containing a fluorescing compound, suitable for inkjet printing applications. The ink formulation comprises at least one organic soluble polyester having at least one near-infrared fluorophore copolymerized therein; an organic solvent selected from C3-6 ketones, C3-6 organic esters, C1-3 alcohols, and combinations thereof; and 1–5 wt % of a binder selected from cellulose ester, condensed phenolic resin, polyketone, polyamide and polyurethane resin and an organic solvent soluble electrolyte.
U.S. Pat. No. 5,958,122 (1999) discloses a printing apparatus in which metered amounts of an ink and a diluting solution (comprising water and at least one water-soluble or water-insoluble organic component) are mixed and the resulting mixed liquid is ejected to a recording medium for jet printing. The water-soluble organic solvent is desirably selected from aliphatic monohydric alcohol, polyhydric alcohol or its derivatives, including lower alcohols, alcohol amines, ketones, amides, ethers, alkylene glycols, polyalkylene glycols, lower alkyl ethers of polyhydric alcohols, and glycerin.
U.S. Pat. No. 5,998,502 (1999) discloses resinous binders suitable for use in CIJ printers which are polyamides soluble in ethanol and having a glass transition temperature of at least 95° C. Because ethanol-based inks are too volatile for use in DOD devices, it is entirely uncertain whether such binders would be soluble in a solvent appropriate for use in a DOD printer.
U.S. Pat. No. 6,010,564 (2000) discloses a jet ink composition suitable for printing images on substrates such as plastics, said composition comprising an organic solvent, a colorant, a cellulose nitrate resin, and a rosin resin. The organic solvent may comprise one or more organic components, selected from ketones, esters, and C1-6 alcohols. While suitable for the CIJ printing method, these inks are likely too volatile for use in DOD devices.
The present invention is directed to overcoming these problems associated with jet inks, providing jet inks particularly suitable for use in DOD printers, and related compositions and methods as described in further detail below.